Method and apparatus for grinding and polishing free-form ophthalmic surfaces

ABSTRACT

A system for polishing and grinding optical surfaces is provides by employing a pad surface that is controllably moved over an affected surface in accordance with control procedures to facilitate a desired surface contour. The system does not require multiple hard master shapes for each desired surface contour but rather is using a limited set of polishing and grinding pad to provide a plurality of conventional and non-conventional surface contours.

FIELD OF THE INVENTION

The present invention relates to grinding and polishing ophthalmic surfaces, such as plastic and glass ophthalmic lenses and glass molds.

BACKGROUND

Conventional devices for grinding and polishing ophthalmic surfaces first apply a hard master shape to an affected surface so as to remove any form error prior to polishing the surface to optical clarity. In the context of an optical laboratory, where prescription lenses are produced, each prescription value requires its own hard master shape. Hence, an optical laboratory typically has to store and maintain a large inventory of hard laps, which may be cumbersome and expensive.

Furthermore, conventional methods are limited to the production of spherical and toric surfaces. Certain specialized Computerized Numerical Control machines may be employed to generate non-conventional surface forms such as aspheric, atoric, and progressive geometries. However, these kind of asymmetric shapes are difficult to grind or polish when employing the above mentioned conventional methods. The equipment currently used for grinding and polishing non-conventional surface forms is from the field of high precision optics. The quality, tolerances, and control in this field are very rigorous in comparison to the field of ophthalmic optics (generally more than one order of magnitude). For this reason, the cost of such high precision equipment is very high, as well as requiring specialized operator training, which essentially renders it unavailable.

Hence, there is a need for a system that allows for automatic grinding and polishing of ophthalmic surfaces, either glass or plastic, which provides a greater surface conformity with respect to the original surface contour, i.e., substantially without any surface deformation, and which obviates the need for an optical laboratory to maintain a large stock of hard master shapes.

There is also a need for providing an apparatus and method which automatically grinds and polishes an ophthalmic surface to geometries that do not provide a symmetry of revolution.

SUMMARY

The present invention provides a reduced cost apparatus and method as compared with equipment presently available from the field of high precision optics. The present invention also provides an apparatus and method for grinding and polishing ophthalmic surfaces which matches or surpasses mechanical specifications required in the area of ophthalmic optics.

In accordance with the present invention, an apparatus for grinding and polishing ophthalmic surfaces is provided. In one embodiment, the resultant surface is provided with a symmetry of revolution. The resultant surface is either concave or convex depending on the desired lens properties. In another embodiment, the apparatus includes a rotating flexible shaping pad, which is substantially smaller than the affected surface. The relatively small size of the flexible pad allows it to deform and contour onto any desirable area on the surface.

The shaping pad is preferably maintained in contact with, and is moved across, the affected surface so as to produce a removal profile which is greatest at the center of pad movement and smallest at the periphery of pad movement.

The apparatus traces a substantially equally-spaced spiraling path across the affected surface at a substantially constant contouring speed. The rotating speed of the pad and its contact pressure are preferably constant as well. The pad support is allowed to pivot in all directions about its rotating axis so as to maintain the plane of said pad orthogonal to the normal of the surface contact point. Preferably, by controlling the movement rate along the contouring path as well as controlling the pad rotation, the material removal rate is controlled.

In one embodiment, the present invention provides an apparatus for grinding or polishing an ophthalmic surfaces. The apparatus includes a surface shaping pad and a rotational pad driver coupled to the pad for maintaining the pad in constant contact pressure with an affected surface and for rotating the pad about a first rotational axis. A position drive means of the apparatus is coupled to the pad driver to controllably move the pad relative to the surface being ground or polished along a substantially spiraling contour path. The contour path is centered at the center of the surface and has parallel spiral arcs, which are spaced at a constant distance along any given radius of the spiraling contour path. The contour path further extends between the contour perimeter and the contour center such that a removal profile is produced along the contour path, which has circular symmetry with peak removal at the center of pad movement and minimal removal at the extremes of pad movement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structure of a surface forming device of the invention;

FIG. 2 is a schematic perspective drawing of the rotating flexible pad drive mechanism; and

FIG. 3 is a schematic drawing of the pad support, which shows in more detail the flexible shaping pad.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary embodiment of a surface forming device, which is constructed in accordance with the invention. The illustrated surface forming device preferably includes a shaping pad 20, a pad drive mechanism 21, a pad drive shaft 22, a pad support 23, and a surface support 34. The shaping pad 20 is preferably mounted onto the pad support 23 which allows for the shaping pad to flexibly move about a universal ball joint. In one embodiment, the flexible rotating shaping pad 20 is controllably moved over a specimen 28 by a pad drive mechanism 21, which includes a pad drive shaft 22. The pad drive shaft 22 is coupled to the pad support 23 so as to transmit rotational movement to the shaping pad 20.

In operation, the pad 20 is controllably moved by the pad drive mechanism 21 in a substantially equally-spaced spiraling path across the surface of the specimen, at substantially constant contouring speed. The pad drive mechanism 21 is advantageously controlled to maintain constant pad pressure against the surface of the specimen 28. The pad drive mechanism 21 rotates the shaping pad 20 about a center axis (R).

In one embodiment, a four-axis positioning arrangement (“X”, “Y”, “Z”, “W”) guides the pad 20 along the desired spiraling path. In this embodiment, the positioning arrangement maintains the pad drive shaft 22 properly aligned with the normal of the surface 28. The positioning arrangement preferably includes the pad drive mechanism 21 and the surface support 34.

The surface support 34 preferably includes a surface holding block 30, a chuck 31, and a holding block drive mechanism 32. The surface holding block 30 is mounted onto the chuck 31 which rotates about a first axis (Z). The pad drive mechanism 21 traverses along a second axis, (X), such that the synchronized motion of the pad 20 along the first axis and the second axis provides a relatively spiraling trajectory with respect to the surface 28. The surface support 34 is adapted to move along a third axis (Y), which is parallel to the first axis (Z). This movement is used to adjust the position of the surface 28 and reciprocate any change in surface height. A fourth axis (W), coupled to the pad drive mechanism 21, allows for pivoting the pad drive mechanism so as to maintain the pad axis of rotation (R) substantially perpendicular to the surface 28.

The pad support 23 allows for the pad 20 to pivot in all directions about the pad rotation axis (R) so as to maintain the geometric plane of the pad oriented orthogonal to the surface contact point.

FIG. 2 illustrates a detailed view of the pad drive mechanism. The illustrated mechanism preferably includes the shaping pad 20, the pad support 23, the universal ball-joint 40, a dowel pin 41, the pad drive shaft 22, and a preloading spring 42. The universal ball-joint 40 allows for the pad support 23 to flexibly pivot about axis of rotation (R). The dowel pin 41 is fastened across the ball joint and allows for the transmission of rotating motion from the pad drive shaft 22 to the pad support 23. The pad support 23 further allows for reciprocating movement along its longitudinal axis (L). To allow for the reciprocating motion, the pad support 23 is preloaded by the spring 42, which allows for producing a constant contact pressure over the surface of the specimen. This reciprocating movement further overcomes any minor deviations emanating from causes such as prismatic blocking of the surface 28 or a slight rotation of the block when mounted onto the chuck 31.

Tables A illustrates parameters used to facilitate the mathematical control procedure used with the control module of the apparatus. The control module comprises a dedicated personal computer (PC), an input-output digital interface board, an operating system for the PC with real-time extensions which allow for a deterministic interrupt handling response better than 10 microseconds, and a control program which executes low level and high level code. TABLE A Tool Path Generation process:  1. Load Machining parameters:    Vt Surface Velocity    Ss Spiral Step    PCT_ERR_Vt Tolerance for Vt fluctuations over 3D surface  2. Load Machine parameters:    VaLim Angular velocity limit    VrLim Radial velocity limit    Dt Base time  3. Load Geometry Parameters: Type of surface Sphere, Asphere, Toric, Atoric, NURBS Surface Parameters Diameter, Curvature(s), Tilt(s), Thickness, etc.  4. Select and prepare Geometry Libraries to     provide geometry functions:    Maximum_Radial_Arc_Length (Surface)    dR_vs_dL (r, dL)    dL_vs_dR (r, dr)    Z_from_Surface (x, y)    Gamma_from_Surface (x, y)    3D_Arc_Length_Over_Spiral_Path (ds, dz, Surface)  Select User Interface special functions  (graphics display):    Operator_Warning (message)  5. Awake and Prepare Low Level section to     receive data from function:    Send_Motion_Deltas (dr, dth, dz, dw) /* Coordinated four axes */  6. Initialize Variables: rM = Diameter/2 rL = Maximum_Radial_Arc_Length (Surface) Tc = ceil(rL/Ss) /* Integer turn count value */ Ss = rL/Tc /* Recalculate Ss */ K = Ss/2π /* Spiral constant for 1 = K*Th */ dth_lim = 2π * VaLim * Dt dr_lim = VrLim * Dt ds_Ref = Vt * Dt TOL_Vk_dS3D = ds_Ref * PCT_ERR_Vt/100.0 r = rM; th = 2π * Tc x = r * cos(th) y = r * sin(th) z0 = Z_Surface(x, y) w0 = Gamma_Surface(x, y)

Both levels of code are executed in a concurrent manner. The low level (real-time) code requires to be serviced by a fast responsive interrupt service routine, so as to be able to read all of the status conditions of the machine, and further being able to send control signals and commands to the different actuators and motors of the machine. High level (user) code provides an operator interface and advantageously uses the resources of the operating system that do not require a real-time response (math co-processor, graphics processor, communications, libraries, etc.).

Said user code is responsible to bring about a valid geometrical description of a tool path for processing an ophthalmic surface. Generating the tool path comprises three steps: a complete and precise description of the ophthalmic surface, a definition of the parameters of the spiraling path to be used across said surface and a description of the kinetic limits of the machine.

The descriptive parameters of the surface include: type of geometry (spherical, toric, aspherical, atoric, progressive, etc.), back and front curvatures of the blank specimen, diameter and thickness of the blank specimen, etc. Non-conventional surface geometries are preferably represented by NURB (non uniform rational B-splines) curves.

The machining parameters of the tool path include: tangential velocity, distance between parallel spiral arcs, rotating speed of polishing pad, etc. The kinetic parameters of each positioning axis include acceleration and velocity limits.

Table B illustrates high level user code which facilitates control of the pad positioning means. TABLE B Main Control Loop: while(th >= 0) { err3D = 0 do { ds = ds_Ref − err3D aux = K * sqrt(th{circumflex over ( )}2 +1) dth = ds/aux if (dth > dth_lim) { /* Keep Angular Velocity Limit */ dth = dth_lim ds = dth * aux ds_Ref = ds Vt = ds/Dt /* New (restricted) Vt */ optn'l: Operator_Warning( “New Vt due to VaLim” ) } dl = K*dth; /* Spiral path over spheroid */ dr = dR_vs_dL( r, dl ) /* Spheroid projection over radial axis */ if (dr > dr_lim) { /* Keep Radial Velocity Limit */ dr = dr_lim dl = dL_vs_dR( r, dr) dth = dl/K /* Reduce angular velocity accordingly */ ds = dth * aux ds_Ref = ds Vt = ds/Dt /* New (restricted) Vt */ optn'l:Operator_Warning( “New Vt due to VrLim” ) } th_aux = th − dth r_aux = r − dr x = r_aux * cos( th_aux ) y = r_aux * sin( th_aux ) z = Z_from_Surface( x, y) dz = z − z0; ds3D = 3D_Arc_Length_Over_Spiral_Path(ds, dz, Surface) err3D = ds3D − ds_Ref } while( |err3D| > TOL_Vk_dS3D ) th = th_aux r = r_aux w = Gamma_from_Surface( x, y ) dw = w − w0 /* Send function performs units conversion between (analytical) User Level and (mechanisms) Low Level */ Send_Motion_Deltas(dr, dth, dZ, dW) z0 = z w0 = w } /* End: Main Control Loop */

Considering the above mentioned parameters, the high level user code generates incremental positioning motions to be executed by the low level real-time code. The motion increments for each positioning axis are simultaneously executed, by said low level code, within a corresponding time increment. The time increments used are preferably constant.

The control module employs coordinated motion between the four-axis positioning mechanism of FIG. 1 to provide a spiraling contour path, as well as properly align the pad 20 with respect to the surface contact point. Preferably, by controlling the movement rate along the contouring path as well as controlling the pad rotation, the material removal rate is controlled. The control module is operative to dynamically adjust the speed of movement of pad 20 along said contour path, as a function of the position of said pad relative to the surface being ground or polished. The control module further regulates the rotating speed of the pad 20 according to the requirements of the process.

Motion control along the four axes X, Y, Z and W is facilitated by an open loop manner, preferably stepping motors, so as to obtain a reduced cost apparatus. However, the present invention contemplates the use of feedback position sensing devices on any axis to provide a higher accuracy closed loop control. The configuration and operation of such feedback sensing device would be apparent to those of ordinary skill in the art. In the illustrated embodiment, the pad drive mechanism 21 is preferably provided with a closed loop speed control, which maintains constant speed pad rotation without substantial fluctuations.

FIG. 3 illustrates a detailed view of the pad support 23 and the flexible shaping pad 20. The shaping pad comprises two layers. A first layer includes a flexible media 50 attached to the pad support 23, which provides necessary deformation to contour onto any desirable area on the affected surface. A second layer includes a polishing or grinding pad 51 attached to a flexible media 50, which produces the surface material removal.

Speed information is preferably provided to the speed control means so as to maintain a constant rotational speed with minimal fluctuations. In one embodiment the system obtains a series of pulses from a Hall Effect sensor. The frequency of these pulses is proportional to the speed of rotation and is used to determined a rotation speed. However, as may be appreciated there are many other methods to obtain a tachometric signal.

The pad support 23 has a spherical surface 52, whereon the flexible pad 20 rests, with a base-curvature value. The present invention contemplates a set of various pad supports, each with a different curvature value, ranging from piano to 10 diopters (concave or convex) in constant increments of 2 diopters. Each individual pad support 23, in combination with a flexible pad 20, will cover a continuous subrange of curvatures to be shaped. The pad support curvature to be applied will depend on the desired lens properties.

The present invention further contemplates the use of pad supports with curvature values greater than 10 diopters (both, concave and convex). However, the limit value of 10 diopters, used at present, allows for the processing of practical prescription values.

Although the present invention was discussed in terms of certain preferred embodiments, the invention is not limited to such embodiments. A person of ordinary skill in the art will appreciate that numerous variations and combinations of the features set forth above can be utilized without departing from the present invention as set forth in the claims. Thus, the scope of the invention should not be limited by the preceding description but should be ascertained by reference to claims that follow. 

1. An apparatus for grinding or polishing an ophthalmic surface, comprising: a surface shaping pad; a rotational pad driver coupled to said pad for maintaining said pad in constant contact pressure with an affected surface and for rotating said pad about a first rotational axis; and a position drive means coupled to said pad driver to controllably move said pad relative to the surface being ground or polished along a substantially spiraling contour path, said contour path centered at the center of said surface, said contour path having parallel spiral arcs, said parallel spiral arcs spaced at a constant distance apart along any given radius of said spiraling contour path, said contour path extending between the contour perimeter and the contour center, such that a removal profile is produced along said contour path having circular symmetry with peak removal at the center of pad movement and minimal removal at the extremes of pad movement.
 2. The apparatus of claim 1 wherein the pad driver includes a pad support for supporting said pad on one end thereof, said pad support being adjustable in a direction generally perpendicular to a surface being ground or polished to maintain constant pressure contact between said pad and said surface.
 3. The apparatus of claim 2 wherein said pad support comprises: a plano pad support; a set of five spherical concave pad supports, each said pad support having a different curvature ranging from −2 diopters to 31 10 diopters, in increments of −2 diopters; and a set of five spherical convex pad supports, each said pad support having a different curvature ranging from 2 diopters to 10 diopters, in increments of 2 diopters.
 4. The apparatus of claim 1 wherein said pad driver includes a spring preloaded reciprocating means to provide a constant pressure between said pad and the affected surface and to dynamically absorb any unexpected small deviations of said surface, said small deviations due to any small mechanical mounting misalignments of said surface.
 5. The apparatus of claim 1 wherein said rotational pad driver includes means to continuously produce location signals indicating the incremental motion of said rotating pad and additionally including speed control means responsive to said location signals to control the speed of said rotational pad driver.
 6. The apparatus of claim 1 wherein said position drive means includes a pivoting drive with a pivoting axis to pivot said pad driver about said pivoting axis for maintaining said pad driver orthogonal to the surface being ground or polished.
 7. The apparatus of claim 6 wherein said position drive means includes a reciprocating drive to reciprocate said pad driver relative to said contour path, such that said pivoting axis is maintained mechanically aligned to the point of contact between said pad and said surface.
 8. The apparatus of claim 1 wherein said position drive means is operative to dynamically adjust the speed of movement of said pad driver along said contour path as a function of the position of said pad driver relative to the surface being ground or polished.
 9. An apparatus for grinding or polishing an ophthalmic surface, comprising: one grinding pad or one polishing pad; a blocking device coupled to the backside of the surface being ground or polished; a rotational chuck holder drive means for holding and rotating said blocking device; a rotational pad driver coupled to said grinding pad or said polishing pad for moving each said pad in a manner which will produce a removal profile having circular symmetry with peak removal at the center of pad movement and minimal removal at the extremes of pad movement; a chuck position drive coupled to said chuck holder drive means for rotating said chuck holder drive means at a controllable velocity about an angular direction; a pad position drive coupled to said pad driver for moving said pad driver at a controllable velocity over a radial direction; and control means for actuating said chuck position drive and said pad position drive at a predetermined velocity in each of said angular direction and said radial direction so that the relative path followed by said pad driver with respect to said chuck holder drive means is a spiraling contour path, said contour path centered at the center of the surface being ground or polished, said contour path having parallel spiral arcs, said parallel spiral arcs spaced at a constant distance apart along any given radius of said spiraling contour path, said contour path extending between the contour perimeter and the contour center of said surface.
 10. The apparatus of claim 9 wherein said grinding pad or said polishing pad includes a polishing surface and said pad driver includes means to dispose said polishing surface in constant pressure contact with a surface to be ground or polished.
 11. The apparatus of claim 10 wherein said pad disposing means includes means to position said polishing surface in contact with and substantially parallel to the surface being ground or polished. 12 The apparatus of claim 9 wherein said constant distance is less than 5% of the minimum dimension of said grinding pad or said polishing pad.
 13. The apparatus of claim 1 wherein said constant distance is less than 5% of the minimum dimension of said surface shaping pad.
 14. A method for grinding an ophthalmic surface, comprising the steps of: rotating a grinding pad having a maximum dimension of less than 25% of the minimum dimension of the surface to be ground with a pad driver; maintaining said rotating grinding pad in constant pressure contact with the surface being ground; moving said pad driver across a contour path so that the center of pad movement follows said contour path to produce a surface removal, being said removal maximum substantially at the center of pad movement and minimum at the extremes of pad movement; and directing the pad driver along said contour path over the surface to be ground, being said contour path a spiraling contour path centered at the center of said surface, said contour path having parallel spiral arcs, said parallel spiral arcs spaced at a constant distance apart along any given radius of said spiraling contour path, said constant distance being less than 5% of the minimum dimension of said grinding pad; said contour path extending between the contour perimeter and the contour center of said surface.
 15. The method of claim 14 further including the control of the speed of rotation of said grinding pad and also including the control of the speed of movement along the contour path of said pad driver so that surface removal can be controlled as a function of both said speed of rotation and said speed of movement.
 16. The method of claim 14 additionally including the use of: one plano pad support; one set of five spherical concave pad supports, each said pad support having a different curvature ranging from −2 diopters to −10 diopters, in constant increments of −2 diopters; and one set of five spherical convex pad supports, each said pad support having a different curvature ranging from 2 diopters to 10 diopters, in constant increments of 2 diopters.
 17. The method of claim 16 wherein each of different said pad supports is used in combination with: an emery media based on aluminum oxide, said media having an average particle size of 9 to 20 microns; a flexible media like neoprene rubber sheet attached to said pad supports, said media having a thickness of typically 3 mm and said media having a typical hardness 30 Shore “A” based on the Durometer scale; and a multi perforated zinc pad with a thickness of 0.5 mm attached to said flexible media, wherein perforations of said zinc pad allow an even distribution of said emery media.
 18. A method for polishing an ophthalmic surface, comprising the steps of: rotating a polishing pad having a maximum dimension of less than 25% of the minimum dimension of the surface to be polished with a pad driver; maintaining said rotating polishing pad in constant pressure contact with the surface being polished; moving said pad driver across a contour path so that the center of pad movement follows said contour path to produce a surface removal, being said removal maximum substantially at the center of pad movement and minimum at the extremes of pad movement; and directing the pad driver along said contour path over the surface to be polished, being said contour path a spiraling contour path centered at the center of said surface, said contour path having parallel spiral arcs, said parallel spiral arcs spaced at a constant distance apart along any given radius of said spiraling contour path, said constant distance being less than 5% of the minimum dimension of said polishing pad; said contour path extending between the contour perimeter and the contour center of said surface.
 19. The method of claim 18 further comprising controlling the speed of rotation of said polishing pad and also including controlling the speed of movement along the contour path of said pad driver so that surface removal can be controlled as a function of both said speed of rotation and said speed of movement.
 20. The method of claim 18 additionally including the use of: one plano pad support; one set of five spherical concave pad supports, each said pad support having a different curvature ranging from −2 diopters to −10 diopters, in constant increments of −2 diopters; and one set of five spherical convex pad supports, each said pad support having a different curvature ranging from 2 diopters to 10 diopters, in constant increments of 2 diopters.
 21. The method of claim 20 applied to glass surfaces wherein each of different said pad supports is used in combination with: a polishing media based on cerium oxide, said media having an average particle size of 1.2 microns and said media having a density of 6 to 8 degree Baumé; a flexible media like neoprene rubber sheet attached to said pad supports, said media having a thickness of typically 3 mm and said media having a typical hardness 40 Shore “A” based on the Durometer scale; and an urethane pad with a typical thickness of 1.5 mm attached to said flexible media.
 22. The method of claim 20 applied to plastic surfaces wherein each of different said pad supports is used in combination with: a polishing media based on aluminum oxide, said media having an average particle size of 1 to 2 microns and said media having a density of 25 to 35 degree Baumé; a flexible media like neoprene rubber sheet attached to said pad supports, said media having a thickness of typically 3 mm and said media having a typical hardness 40 Shore “A” from the Durometer scale; and a napped poromeric structure pad with a typical thickness of 1.5 mm attached to said flexible media, wherein said poromeric structure creates a pumping action under the compression of said pad, said pumping action enhancing the flow of said polishing media. 